Photo lIbrary problem

When I open a library and select export to move to another folder...nothing happens. No dialogue box...nothing.

when you open a Photos Library in Photos you should be able to export selected photos. It is not normal that no dialog box appears. Could it be that the dialog is hidden behind another window?

Restart your Mac and Photos, then try to export with no other application running.

Do you have plenty of free storage on your Mac? Is your Photos Library stored locally on yourMac or on an external volume?

Are you using iCloud Photos?

You are selecting the thumbnails of pictures you want to export, correct?

then going file>export?

See here for the official methods for merging photo libraries.

https://support.apple.com/en-gb/HT209528

These are not ideal. If you use iCloud there is a better method of merging via iCloud.

Another alternative is to use power photos (a paid app) which can copy images from one library to another.

https://www.fatcatsoftware.com/powerphotos/

That's not the correct way to merge libraries. The quicker and easiest is to use the paid version of PowerPhotos to merge the libraries according to this PowerPhotos help file: Merging libraries.

The other way is to use the iCloud Library. That requires you to purchase enough iCloud storage to house both libraries. The process can take up to weeks depending on the size of the libraries.

Exporting the photos out of one library and importing into the other will lose all metadata you've added and any albums, etc. that you've created. The other methods do not.